Autosomal dominant hypophosphatemic rickets (ADHR) is a renal phosphate wasting disorder resulting from Fibroblast Growth Factor-23 (FGF23) missense mutations. These mutations prevent cleavage of FGF23 into inactive fragments. ADHR has a variable timing of penetrance and waxing and waning of FGF23 concentrations which we have demonstrated relates to iron concentrations in cross-sectional and longitudinal studies, such that only subjects with iron deficiency developed high FGF23 concentrations. In light of our novel findings, we propose testing the following hypothesis: Iron repletion with low dose oral iron will decrease C-terminal and intact FGF23 concentrations in ADHR patients having baseline iron concentrations in the deficient and insufficient range and rescue the hypophosphatemic phenotype. We propose the following specific aim: 1. Perform a pilot study in ADHR patients having low serum iron concentrations to determine if increasing serum iron concentrations above 100 mcg/dl results in decreasing intact FGF23 (primary endpoint) and C-terminal FGF23 concentrations and normalizes serum phosphorus and TMP/GFR. Successful completion of the proposed pilot study will enable the investigative team to choose an optimal dosing regimen; determine the kinetics of the iron/FGF23/phosphate relationship; determine which measure of iron status best reflects the relationship between iron and FGF23 (serum iron versus indices of iron stores or anemia); and determine the iron dosing and feasibility for performing a larger multicenter trial to treat ADHR with iron.